Some of the cheap RDS encoders use RS232.
So when you buy one you may need to get yourself an adaptor to convert USB to rs232 if your computer doesn't have a serial/RS232 port. They are usually a simple cable with usb connector at one end and 9-way D-Type at the other.

RDS using a audio DAC is not possible unless your sample rate is atleast 2x the nyquist of the RDS subcarrier which is 2x 57KHz = 114KHz sample rate at a bare minimum, and even then there will still be issues, ideally the RDS would be 80% of the nyquist at s minimum to not avoid any issues caused by the LPF and resconstruction filter of the DAC.